This invention relates to a signal processing circuit for an electronic weighing apparatus which not only amplifies analog weight signals but also efficiently attenuates the noise in these signals caused by mechanical vibrations, and more particularly to such a signal processing circuit with an extremely small delay in response due to its filters. This invention further relates to a weighing apparatus incorporating such a signal processing circuit.
Weighing is often effected electronically nowadays rather than mechanically. An electronic weighing apparatus typically uses a load cell of a known kind affixed on one side to the main frame of the apparatus and on the other side through a bracket to a weigh hopper with a gate. Combinational weighing systems disclosed in U.S. Pat. No. 4,398,612 and U.S. patent application Ser. No. 762,722 filed Aug. 5, 1985 and assigned to the present assignee are examples in which use is made of such weighing apparatus with load cells. Such a load cell is adapted to output an analog signal indicative of the value of the gravitational force on the cell. A circuit for processing such signals in the case of a combinational weighing system with a plurality of article batch handling units is illustrated in FIG. 19 wherein weighing devices 1 such as load cells attached to weigh hoppers (not shown) output signals to associated amplifier circuits 2. Behind each of these amplifier circuits 2 is a set of three low pass filters 3-1, 3-2 and 3-3 for attenuating noise in inputted signals. Numeral 4 indicates a multiplexer for selectively outputting the weight signals from the individual article batch handling units; numeral 5 indicates a zero-point adjustment circuit for subtracting from the weight signals the voltage corresponding to the initial load such as the weight of the unloaded hopper; numeral 6 indicates a device for controlling the level of adjustment by the zero-point adjustment circuit 5; numeral 7 indicates a sample-and-hold circuit of a known kind; numeral 8 indicates an analog-to-digital converter; numeral 9 indicates a device for controlling a reference voltage for the analog-to-digital converter 8 for each weighing device in order to keep its span at a predetermined level; and numeral 10 indicates a computer which performs arithmetic operations for combinations of the weight values obtained from the weighing devices and selects a combination on the basis of a predetermined criterion in view of a given target weight value. Weigh hoppers associated with the combination thus selected are discharged in response to a signal outputted from this computer.
Since the major features of electronic weighing and, in particular, of combinational weighing are great accuracy and high throughput, a signal control circuit therefor must be correspondingly efficient. A load cell typically forms an oscillating system and continues to oscillate, outputting a waveform as shown in FIG. 6(A) wherein t.sub.1 represents a period in which small oscillations caused by the vibrations of the supporting frame are outputted. When the gate of the associated weigh hopper is mechanically operated by an external force, the load cell reacts as shown in the period t.sub.2. When the weigh hopper is released from the influence of external forces, the oscillations in the outputted signal are attenuated gradually as shown in the period t.sub.3. In summary, FIG. 6(A) may be interpreted as representing a typical signal waveform when a loaded weigh hopper is discharged and immediately reloaded.
If a weight signal containing noise from various sources as shown in FIG. 6(A) is passed through a series of appropriately chosen low pass filters 3-1, 3-2 and 3-3 as shown in FIG. 16, a waveform depicted in FIG. 6(B) may be obtained with high-frequency components attenuated or effectively removed. A comparison between FIGS. 6(A) and 6(B) shows that the signal which passed through the low pass filters takes a fairly long time to become stable. In other words, effects of a new article batch dropped into a weigh hopper remain for a long time and hence an analog signal indicative of, or proportional to the true weight value does not become available for a long time.
In another aspect of the technology of electronic weighing, there is the well known problem of drifts in output signals due to variations, for example, in the source voltage and tempeature. Since such drifts inevitably cause errors in the computed final weight values, so-called zero-point adjustments must be performed frequently, but the zero-point adjustment of a weighing apparatus can generally be performed only when the associated weigh hopper is empty. When a combinational weighing system is in operation, however, periods during which a weigh hopper is empty are extremely short in duration and a stable zero-point is generally not obtainable in such a short period. Moreover, span adjustments of weighing apparatus must also be performed in order to improve the accuracy of measured values. Since it is usually necessary to place a standard weight in a weighing device, span adjustment could be effected only at times of periodic inspection. In still another aspect, self-check circuits have been considered for checking errors in a signal processing circuit so as to eliminate errors in measurements, but such self-checking operations must also be completed quickly between consecutive cycles of weighing and combinational computations.